Intelligent field shaping for magnetic speed sensors

ABSTRACT

The present disclosure provides for techniques to improve the sensitivity of magnetic sensor systems. One embodiment of a magnetic sensor system includes a magnetic biasing body comprised of a hard magnetic material and including a recess therein. The recess corresponds to a magnetic flux guidance surface of the magnetic biasing body. The magnetic sensor system also includes a magnetic sensing element arranged in or proximate to the recess. A magnetic flux concentrator, which is made of a soft magnetic material, is disposed in the recess between the magnetic flux guidance surface and the magnetic sensing element. Other techniques are also described.

BACKGROUND

In many applications, it is useful to detect changes in magnetic fieldto track translational motion, rotational motion, proximity, speed andthe like. GMR (Giant Magnetoresistance), AMR (AnisotropicMagnetoresistance), TMR (Magneto Tunnel Effect), and CMR (ColossalMagnetoresistance) sensors, as well as Hall-effect sensors, aredifferent types of magnetoresistive sensors that can measure changes inmagnetic field.

Typically, a magnetoresistive sensor has a high sensitivity in aso-called “working range” of the sensor. In this context, “highsensitivity” means that a small change in magnetic field applied to thesensor leads to a large change in resistance as measured by the sensor.Outside of the working range, unfavorable behavior of themagnetoresistive effect, such as saturation for example, limits the useof the sensor for many applications. Although existing magnetoresistivesensor systems are adequate in many respects, existing sensor systemssuffer from a limitation in they have been unable to maximize thesensitivity and/or working range of the sensors.

SUMMARY

The following presents a simplified summary of the invention in order toprovide a basic understanding of some aspects of the invention. Thissummary is not an extensive overview of the invention, and is neitherintended to identify key or critical elements of the invention nor todelineate the scope of the invention. Rather, the purpose of the summaryis to present some concepts of the invention in a simplified form as aprelude to the more detailed description that is presented later.

The present disclosure provides for techniques to improve thesensitivity of magnetic sensor systems. One embodiment of a magneticsensor system includes a magnetic biasing body comprised of a hardmagnetic material and including a recess therein. The recess correspondsto a magnetic flux guidance surface of the magnetic biasing body. Themagnetic sensor system also includes a magnetic sensing element arrangedin or proximate to the recess. A magnetic flux concentrator, which ismade of a soft magnetic material, is disposed in the recess between themagnetic flux guidance surface and the magnetic sensing element.

The following description and annexed drawings set forth in detailcertain illustrative aspects and implementations of the invention. Theseare indicative of but a few of the various ways in which the principlesof the invention may be employed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a three-dimensional depiction of a magnetoresistive sensorthat includes a magnetic flux concentrator in accordance with oneembodiment.

FIG. 2 depicts a cross-sectional view of FIG. 1's magnetoresistivesensor with magnetic field lines superimposed thereon.

FIG. 3A shows a chart illustrating magnetic swing signal amplitude as afunction of distance between a magnetic sensor and a magnet for variousmagnetic sensor arrangements.

FIGS. 3B-3D show three different magnetic sensing configurations300B-300D, respectively, illustrating various tradeoffs.

FIGS. 4A and 4B show a three-dimensional depiction and a cross-sectionaldepiction, respectively, of a magnetoresistive sensor having a pyramidalflux guidance surface in accordance with some embodiments.

FIGS. 5A and 5B show a three-dimensional depiction and a cross-sectionaldepiction, respectively, of a magnetoresistive sensor having a conicalflux guidance surface in accordance with some embodiments.

FIGS. 6-8 show top views of magnetoresistive sensors in accordance withsome embodiments.

DETAILED DESCRIPTION

The present invention will now be described with reference to thedrawings wherein like reference numerals are used to refer to likeelements throughout, and wherein the illustrated structures are notnecessarily drawn to scale. Nothing in this detailed description (ordrawings included herewith) is admitted as prior art.

FIG. 1 shows a perspective view of a magnetic sensing device 100 inaccordance with some embodiments. The magnetic sensing device 100includes a back bias magnet 102 having a recess 104 therein. The recess104 corresponds to a flux-guidance surface 106, which is illustrated asbeing comprised of four planar surfaces that form an inverted pyramidalin shape in FIG. 1. A magnetic sensing element 108 is arranged proximateto the flux-guidance surface 106 to sense the magnetic field induced bythe back bias magnet 102 and other nearby magnets. Crane-likeprotrusions 118 a, 118 b can also extend upward from two or more sidesof the back bias magnet 102.

The magnetic sensing element 108 may comprise a semiconductor chiphaving at least one magnetoresistive or Hall sensor element providedthereon. The magnetic sensing element 108 may be a GMR, MTR, CMR, AMRelement or any other form of magnetoresistive sensor element. In someembodiments, the magnetic sensing element 108 may have two sensingelements provided in a gradiometer arrangement, and/or may supply adifferential signal from at least two sensing elements therein. In someembodiments, the magnetic sensing element 108 includes a plurality ofmagnetoresistive sensing elements arranged in a Wheatstone bridgeconfiguration. Whatever the particular sensor configuration, themagnetic sensing element 108 is sensitive to changes in magnetic fieldin the x and/or y directions, which may be herein referred to as lateralcomponents of the magnetic field. In contrast, the magnetic sensingelement 108 is generally unaffected by magnetic field changes in thez-direction, which may be herein referred to as a vertical component ofthe magnetic field.

To provide a larger amplitude swing for the magnetic sensing element 108relative to existing implementations, the magnetic sensing device 100includes a magnetic flux concentrator 110. The magnetic fluxconcentrator 110 is arranged between magnetic sensing element 108 andflux guidance surface 106.

In some embodiments, the magnetic flux concentrator 110 extendscontinuously between flux guidance surface 106 and magnetic sensingelement 108 (e.g., completely fills recess below magnetic sensingelement 108). However, in other implementations, the magnetic fluxconcentrator can also be spaced apart from the flux-guidance surface 106and/or sensing element 108 (e.g., only partially fills recess belowmagnetic sensing element 108).

In some embodiments the magnetic flux concentrator 110 is a softmagnetic material, while the back bias magnet 102 is a hard or permanentmagnetic material. In other embodiments, the magnetic flux concentrator110 and back bias magnet 102 are both hard magnetic materials or areboth soft magnetic materials. Although the terms “hard” and “soft” arerelative terms, it will be appreciated that a “hard” magnetic materialis difficult to magnetize relative to a “soft” magnetic material. In asimilar manner, the hard magnetic material is difficult to de-magnetizerelative to a “soft” magnetic material. Hence, hard magnetic materialsare well suited for permanent or long-lasting magnets, and soft magneticmaterials are only transiently magnetized. “Hard” magnetic materialshave high coercivity, whereas “soft” magnetic materials have lowcoercivity. Examples of soft ferromagnetic materials include but are notlimited to: annealed iron, low carbon steel and plastically molded ironparticles. Examples of hard ferromagnetic materials include, but are notlimited to: alnico, ferrite, and rare earth magnets.

As shown in FIG. 2, the flux guidance surface 106 guides magnetic fieldlines 112 so that, absent other magnetic fields, the magnetic fieldlines 112 are substantially aligned with the z-axis at the magneticsensing element 108. The magnetic flux concentrator 110 can induce someadditional “bend” to the magnetic field lines 112. Because the magneticsensing element 108 is sensitive only to x-and/or y-component changes inthe magnetic field, the flux guidance surface 106 in combination withthe magnetic flux concentrator 110 facilitates the magnetic sensingelement 108 switching between a magnetically unsaturated state (neutralresistance when magnetic field lines are aligned to z-axis as shown inFIG. 2) and a magnetically saturated state (high or low resistance whenmagnetic field lines are “twinged” to have x- or y-components at themagnetic sensing element 108).

For example, as shown in FIG. 2, when a rotary element 114 having aplurality of magnets 116 with alternating magnetization arranged aroundits outer circumference, rotates about its central axis; the resultantmagnetic field lines 112 due to the magnets 116 correspondingly rotateto change the magnetic field signal detected by the magnetic sensingelement 108. At the angular position illustrated in FIG. 2, for example,the magnetic field lines at the magnetic sensing element 108 are alignedto the z-axis. Because the magnetic sensing element 108 is sensitive tomagnetic field components only along the x-axis and/or y-axis, thisangular position results in the magnetic sensing element 108 recording alow amplitude (e.g., near-zero magnitude) magnetic field signal. As therotary element 114 rotates, however, the magnetic field lines 112 are“twisted” so the magnetic sensing element 108 sees a non-zero magneticx-component and/or y-component. By tracking the time-dependent magneticfield as the rotary element 114 rotates about its axis, the magneticsensing element 108 can provide an output signal whose amplitude isindicative of the angular position of the rotary element 114.

Ideally, the change in amplitude between the various angular positionsis as large as possible, as this helps to provide better angularmeasurement capabilities. Because magnetic field magnitude decreaseswith distance from a magnet, the inventors have appreciated thatmagnetic sensing elements in previous approaches, which included only anair gap and not magnetic flux concentrator 110, were unduly far from themagnets 116. In FIG. 2's embodiment, the magnetic flux concentrator 110causes the magnetic field lines 112 to exhibit some additional“curvature” or “twinge” relative to a simple air gap, such that thedistance d between sensor 108 and magnets 116 is less than that ofprevious implementations while still limiting offset drift. In this way,FIG. 2's magnetic sensing element 108 is able to experience a greateramplitude swing with smaller offset drift than previously achievable.Thus, due to the magnetic flux concentrator 110, the overall amplitudeswing measured by the magnetic sensing element 108 is greater thanpreviously achievable, which enables more accurate magnetic fieldmeasurements by limiting saturation more than previous approaches.

FIG. 3A shows a series of curves illustrating the magnetic signal swingfor various magnetic sensor arrangements. For example, curve 302 showsan existing implementation where a magnetic sensing element is arrangedover a rare-earth back bias magnet with only an air gap therebetween(i.e., no magnetic flux concentrator is included). As shown, as thedistance between the magnetic sensing element and the back bias magnetis decreased in region 310, the signal swing tends to increase. Thiscurve 302 for the rare-earth element back bias magnet represents goodperformance relative to an implementation represented by curve 304 wherean air gap separates a ferrite back bias magnet from a magnetic sensingelement. Unfortunately, although the rare-earth back bias magnet ofcurve 302 provides fairly good performance, it is expensive due to thescarcity of rare-earth materials.

Curve 306 represents the signal swing performance when a magnetic fluxconcentrator (instead of a simple air gap) is inserted between a ferriteback bias magnet and a magnetic sensor, such as previously illustratedin FIG. 1 for example. Because ferrite is relatively inexpensive, such asolution is relatively cheap compared to a sensor system where a rareearth magnet is used. Further, as can be seen from curve 306, thissolution also offers a fairly large signal swing. Thus, this solutionoffers particularly good tradeoffs between cost and performance.Further, if cost is not a concern, a flux concentrator can be used witha rare-earth magnet (see e.g., curve 308) to achieve extremely highperformance.

FIGS. 3B-3D show three different magnetic sensing configurations300B-300D, respectively, illustrating various tradeoffs. The firstmagnetic sensing configuration 300B (FIG. 3B) shows an example where aback-bias magnet 310 is spaced apart from a zero-magnetic field location312 by a height h₁, representing a “deep” cavity in the back bias magnet310. Because this zero-magnetic field location 312 is where the biggestmagnetic signal swings occur, a magnetic sensor (not shown) can bepositioned at this zero-magnetic field location 312 to detect changes inmagnetic field (e.g., due to a magnetic wheel).

In FIG. 3C's magnetic sensing configuration 300C, the cavity depth hasbeen reduced to h₂ (h₂<h₁), such that the non-zero magnetic fieldlocation 314 is positioned deeper in the cavity within back-bias magnet316, relative to FIG. 3B's magnetic sensing configuration. Because thecavity depth is more “shallow” in FIG. 3C, the magnetic signal swings inthe x- and/or y-directions at the zero-magnetic field location 314 willtend to be larger than in FIG. 3B's example, which promotes reliabledetection of changes in magnetic field. Unfortunately, because thezero-magnetic field location 314 is located within the cavity, it ismore difficult to get a magnetic field sensor positioned reliably atthis location compared to FIG. 3B's embodiment.

FIG. 3D depicts a particularly advantageous magnetic sensingconfiguration 300D having a back-bias magnet 318 with a cavity, whereina magnetic flux concentrator 320 is positioned in the bottom of thecavity. The back bias magnet 318 and magnetic flux concentrator 320 arecollectively closer in proximity to a zero-magnetic field location 322than in FIG. 3B, which promotes large signal swings and high precisionmagnetic field measurements. At the same time, the zero-magnetic fieldlocation 322 is aligned on-plane with the upper surfaces 324A, 324B ofprotrusions 326A, 326B. Thus, this arrangement 300D provides streamlinedmanufacturing in that a magnetic sensor (not shown) can be reliablypositioned on plane with protrusion upper surfaces 324A, 324B—whichpromotes ease of manufacture. Further, at this position, the magneticsensor 300D is subject to large magnetic signal swings—which promotesreliable magnetic field sensing. In addition, when the magnetic fieldconcentrator 322 is made of a soft magnetic material (e.g., ferritematerial), the cost of the overall sensing arrangement 300D can bereduced relative to embodiments where a back-bias magnet is made whollyof the hard magnetic materials, such as a rare earth element, forexample.

It will be appreciated that a number of variations are contemplated asfalling within the scope of the present disclosure. For example,referring briefly back to FIG. 1, although the magnetic sensing element108 is illustrated as being centered in the recess 104 with zero x- andy-field components; in other embodiments the magnetic sensing element108 may be off-center or outside the recess in order to reduce thesensitivity. This may for example be achieved by having the magneticsensing element 108 moved away from the region with zero x- andy-component along the guide or support formed by protrusions 118 a and118 b. Further, an angle of inclination of the flux guidance surfaceshaped by the recess may in one embodiment be selected from the rangebetween 5° and 65° when taken from the x-axis. In one embodiment, theangle of inclination may be selected between 5° and 40°. In oneembodiment, the angle of inclination may be selected between 5° and 20°.

In embodiments described below in more detail, the recess 104 and/ormagnetic flux guidance surface 106 may have a pyramid form, a conicalform or a polyhedron form. A number of variations are now described withrespect to the remaining figures. It will be appreciated that theillustrated and described embodiments can be combined in any number ofways, and are not limited to the illustrated embodiments.

FIGS. 4A-4B show an embodiment wherein the sensor 108 is placed on asupport structure (e.g., PCB or semiconductor substrate) over fluxguidance surfaces 106 a and 106 b. For purposes of clarity, FIG. 4Ashows only back bias magnet 102, while FIG. 4B shows back bias magnet102 with magnetic sensing element 108 and magnetic flux concentrator110. The flux guidance surfaces 106 a and 106 b are provided at thelateral border of the back bias magnet 102. The magnetic fluxconcentrator 110 is arranged between the flux guidance surfaces 106 a,106 b and the sensor 108. Although FIG. 4B shows an air-gap 120 betweenthe magnetic flux concentrator 110 and sensor 108, the recess 104 may becompletely filled with the flux concentrator 110 in other embodiments,thereby “squeezing out” air gap 120.

FIGS. 5A-5B show an embodiment wherein the recess 104 penetrates theback bias magnet 102 in the vertical direction to form a hole in the topand bottom surfaces of back bias magnet 102. The sensor 108 is placed inthe embodiment according to FIG. 5B completely within an IC package 122,which may be magnetizable material or non-magnetizable. FIG. 5B showsthe recess 104 to have an inclined surface 106 with respect to thevertical direction such that the width in x-direction decreases towardsthe sensor 106. However, other embodiments may provide otherinclinations or no inclination with respect to the vertical direction.The flux concentrator 110 is illustrated as partially filling the recess104, wherein the amount of filling can depend on the particularimplementation.

Having now described cross-sectional views of embodiments, FIGS. 6-8show exemplary top views which may apply to each of the embodimentsdescribed with respect to FIGS. 1, 2, 4, and 5. For purposes ofsimplicity, these top views show only the back bias magnet, and do notshow a magnetic flux concentrator formed thereover. However, it will beunderstood that in practical applications, the magnetic fluxconcentrator is formed over the back bias magnet.

FIG. 6 shows a top-view of the back bias magnet 102 wherein the recess104 has a pyramid shape or a shape of half of an octahedron. While FIG.6 shows the pyramid shape in top-view to have a quadratic form, it maybe noted that also a rectangle form with extensions in x and y-directionbeing different may be provided in embodiments.

FIG. 7 shows a top-view of the back bias magnet 102 wherein the recess104 has the shape of one half of a polyhedron with 16 surfaces. Inembodiments, the recess 104 may have the form of regular polyhedrons orparts of regular polyhedrons.

FIG. 8 shows a top-view of the back bias magnet 102 according to afurther embodiment where the recess 104 has a circular form withdecreasing radius when moved along the vertical line. In a furtherembodiment, the recess 104 may have the form of a truncated cone.

Each of the top view forms shown and described with respect to FIGS. 6-8may be have one of the cross-sectional views shown and described withrespect to FIG. 1, 2, 4, or 5. For example, the protrusions shown inFIGS. 1 may be provided for the pyramid shape as shown and describedwith respect to FIG. 6, for the polyhedron shape as shown and describedwith respect to FIG. 7 or for the cone shape as shown and described withrespect to FIG. 8.

Each of the embodiments shown in FIGS. 6 to 8 has in the x-y plane asymmetric structure with a defined center of symmetry. For suchstructures, the region of zero or substantially zero magnetic x- andy-components includes the center of symmetry. However, other embodimentsmay have a non-symmetric structure when viewed from the top.

In one embodiment, the back bias magnet 102 can be manufactured bymolding hard magnetic and/or soft magnetic material. The molding of theback bias magnet 102 with its geometrical shape can be done with moldtools directly on top of the sensor 108 as an additional packaging step.In some embodiments, the back bias magnet 102 and the sensor 108 may beintegrated. In some embodiments, the back bias magnet 102 and the sensor108 may be integrated within a common package which may be formed bymolding over the back bias magnet 102 and the sensor 108. In someembodiments, the back bias magnet 102 can be assembled on the sensor 108with the usage of adhesive glues or only with mechanical clampingmechanism. In some embodiments, the back bias magnet 102 can beassembled with the sensor 106 and fixed with a mold material that ismolded around the whole system for example in a thermoplast injectionmold process.

Although various embodiments for manufacturing a magnetic sensor havebeen discussed and illustrated above in the context of magneto resistivesensors, the manufacturing methods and other concepts are alsoapplicable to other types of magnetic sensors. In regard to the variousfunctions performed by the above described components or structures(blocks, units, assemblies, devices, circuits, systems, etc.), the terms(including a reference to a “means”) used to describe such componentsare intended to correspond, unless otherwise indicated, to any componentor structure which performs the specified function of the describedcomponent (e.g., that is functionally equivalent), even though notstructurally equivalent to the disclosed structure which performs thefunction in the herein illustrated exemplary implementations of theinvention. In addition, while a particular feature of the invention mayhave been disclosed with respect to only one of several implementations,such feature may be combined with one or more other features of theother implementations as may be desired and advantageous for any givenor particular application. Furthermore, to the extent that the terms“including”, “includes”, “having”, “has”, “with”, or variants thereofare used in either the detailed description and the claims, such termsare intended to be inclusive in a manner similar to the term“comprising”.

What is claimed is:
 1. A magnetic sensor system, comprising a rotaryelement including a plurality of magnets arranged about an outercircumference thereof; a magnetic biasing body spaced apart from therotary element and comprised of a hard magnetic material and including arecess therein, wherein the recess includes sidewalls and a lowersurface and corresponds to a magnetic flux guidance surface of themagnetic biasing body; a magnetic sensing element arranged to sensechanges in magnetic field orientation at a location in or proximate tothe recess; and a soft magnetic material disposed in the recess over thelower surface and between the sidewalls, the soft magnetic materialarranged between the magnetic flux guidance surface and the magneticsensing element, the soft magnetic material being configured to reduce amagnetic field due to the magnetic biasing body at the location wherethe changes in magnetic field orientation are sensed by the magneticsensing element.
 2. The sensor system of claim 1, wherein the softmagnetic material is spaced apart from the magnetic sensing element. 3.The sensor system of claim 1, wherein the flux guidance surface has aninverted pyramidal shape.
 4. The sensor system of claim 1, wherein theflux guidance surface comprises three or more inclined planar surfacesthat intersect at a junction region.
 5. The sensor system of claim 1,wherein the flux guidance surface is conical in shape.
 6. The sensorsystem of claim 1, wherein the flux guidance surface is defined betweena first circular or rounded opening having a first area and a secondcircular or rounded opening having a second, different area, wherein atapered sidewall connects the first circular or rounded opening and thesecond circular or rounded opening.
 7. The sensor system of claim 1,wherein the hard magnetic material comprises a ferrite material.
 8. Thesensor system of claim 1, wherein the soft magnetic material comprises aferrite material.
 9. The sensor system of claim 1, wherein the hardmagnetic material comprises a rare earth material.
 10. The sensor systemof claim 1, wherein the soft magnetic material is conformally disposedover the flux guidance surface.
 11. The sensor system of claim 1,wherein the soft magnetic material is disposed over less than all of theflux guidance surface.
 12. The sensor system of claim 1, wherein themagnetic biasing body comprises protrusions extending therefrom, whereinthe protrusions have respective upper surfaces that lie along a plane.13. The sensor system of claim 12, wherein the magnetic sensing elementis arranged on a support structure bridging the recess and coupled tothe upper surfaces.
 14. The sensor system of claim 13, wherein themagnetic sensing element is positioned on or adjacent to the plane. 15.The magnetic sensor system of claim 1: wherein the recess in themagnetic biasing body extends downwardly from an uppermost surface ofthe magnetic biasing body, between sidewall portions of the magneticbiasing body, and terminates at an upper surface of a lower portion ofthe magnetic biasing body; and wherein a thickness of the soft magneticmaterial is configured to alter the magnetic field due to the magneticbiasing body so the altered magnetic field has a zero magnetic field atthe location, and where the location lies on a plane corresponding tothe uppermost surface of the magnetic biasing body.
 16. The magneticsensor system of claim 1: wherein the recess in the magnetic biasingbody extends downwardly from an uppermost surface of the magneticbiasing body, between sidewall portions of the magnetic biasing, andterminates at an upper surface of a lower portion of the magneticbiasing body; and wherein the thickness of the soft magnetic material isconfigured to alter the magnetic field due to the magnetic biasing bodyso the altered magnetic field has a zero magnetic field at the location,and wherein the location is in the recess over an upper surface of thesoft magnetic material.
 17. An apparatus, comprising: a rotary elementincluding a plurality of magnets arranged about an outer circumferencethereof; a magnetic biasing body spaced apart from the rotary elementand including a recess therein, wherein the recess includes sidewallsand a lower surface and corresponds to a magnetic flux guidance surfaceof the magnetic biasing body; a magnetic sensing element configured tosense changes in magnetic field orientation at a location between therotary element and the magnetic biasing body and in or proximate to therecess; and a magnetic flux concentrator disposed in the recess over thelower surface and between the sidewalls, the magnetic flux concentratorarranged between the magnetic flux guidance surface and the magneticsensing element, wherein the magnetic flux concentrator is configured toalter a magnetic field due to the magnetic biasing body so the alteredmagnetic field has a zero magnetic field at the location where thechanges in magnetic field orientation are sensed by the magnetic sensingelement.
 18. The apparatus of claim 17, wherein the magnetic biasingbody comprises protrusions extending from a base region of the magneticbiasing body towards the rotary element, wherein the protrusions haverespective upper surfaces that lie along a plane.
 19. The apparatus ofclaim 18, wherein the magnetic sensing element is arranged on oradjacent to the plane and changes its resistance proportional to changesin a magnetic field component in the plane, but is insensitive tochanges in a magnetic field component perpendicular to the plane. 20.The apparatus of claim 18, wherein the magnetic biasing body comprisesmagnetic material having a magnetic hardness that is greater than thatof the magnetic flux concentrator.